The drive toward higher performance electronic and opto-electronic devices requires the use of materials of highly tailored electrical and optical properties. The materials are usually crystalline alloys deposited in complex stacking arrangements. While the technologies for depositing the stacked layers have been greatly improved, there is need for more innovation in the etching technologies which are specifically designed for these multi-component alloys. Furthermore, these advanced devices usually rely on active areas which are only a few monolayers thick, thus requiring precise control over etch rates for reproducible end point detection. To overcome this problem, it is usually necessary to include etch stop layers in the device structure, complicating the growth and adding additional steps in the fabrication process. Several other approaches are possible. The thickness of the film may be measured optically by interferometry or other optical means. The presence of a reaction product or the absence of a reacting chemical species can also be monitored and used to determine when the film etching is complete. Detection in these latter two cases can be either by optical or by mass spectroscopy methods, but an interface between different chemical compositions is necessary for detection. When commercially available differentially pumped mass spectrometers are used, very complex spectra are normally obtained which are difficult to interpret.
The use of enriched isotopes is well known in a variety of fields of science. There is a class of applications called "isotopic engineering," where both precise control of isotope composition and large enough quantities to allow use of the isotopes in multiple samples are possible. Although the isotopes may be used in a production process, extremely small quantities may be used. A recent review includes a discussion of scientific knowledge gained through studies of isotopically controlled semiconductor crystals and a discussion of future possibilities offered through isotope control of a wide range of semiconductor materials (Haller, "Isotopically Engineered Semiconductors." i J Appl. Phys. 77 (7), 1995). The author of this review discusses superlattices grown with enriched isotopes and the doping of isotope layers using Neutron Transmutation Doping (NTD). The availability of highly enriched isotopes of semiconductor elements has been a bottleneck in the development of their use. However, greater availability is increasingly apparent and it is possible to use only an increase of a certain isotope (in contrast to greater than 90% pure) or extremely thin epilayers to achieve practical results. The author estimates that the prices of isotopes will drop from the current prices of more than $1,000 per gram for pure isotopes to a range less than $100 per gram when substantial demand for the isotopes arises. This latter price is comparable to current prices for high purity source materials for thin film growth. The materials will thus become even more feasible for production-scale device fabrication.
There is a body of work using isotopically tagged precursors in epitaxial growth in both group IV and in groups III-V compounds. It is clear that the science and technology of chemical compound synthesis is capable of delivering isotopically tagged materials with the purity and reproducibility necessary for production-scale, semiconductor thin film growth. Heterostructures with submonolayer sharp interfaces and precise doping profiles have been reproducibly demonstrated. To date, many deposition methods have been shown to be efficient growth technologies for such heterostructures.
Methods are needed for improving device fabrication processes employing gas phase etching. One such improvement needed is an end point detection during etching of heterostructures. This might include a signal to a process controller to cause the process to go into a slower etch mode or shut down in a preprogrammed time interval during an etch process. There is also a need for real time markers for calibration of the rate of an etching process. This is particularly important in compound semiconductors where etch chemistries are highly complex and etch rates vary significantly with alloy composition and dopant levels. A method is needed which does not affect etch chemistry and can be used when no difference in chemical composition exists. The process should be workable at an interface where only a slight composition variation exists or where only a change in dopant level is present. It also should be applicable in processes using selective area chemical beam epitaxy growth followed by chemical beam etching. The method should require only extremely thin layers, so that it has negligible effect on epitaxial processes or requires minimum amounts of expensive isotopically pure chemical materials. There is also a need for methods to assess the etch uniformity of processes. Similarly, a method is needed to assess the uniformity of an epitaxial growth process. Sensitivity sufficient to allow operation of the process when extremely low surface area percentage area openings (vias) exist during etching is needed, using labeled layers that can be detected with very high signal to noise ratios, such that the process will be applicable to design geometries even below 0.33 micron and etch areas below 1 per cent of total wafer area.